LOW-TEMPERATURE HOPPING CONDUCTION IN NEUTRON TRANSMUTATION DOPED ISOTOPICALLY ENRICHED GE-70 - GA SINGLE-CRYSTALS

The temperature dependence of variable range hopping resistivity rho i n neutron transmutation doped (NTD) isotopically enriched Ge-70:Ga sam ples is reported. Five samples with compensation ratios K less than 0. 001 and Ga concentrations between 3x10(16) and 1.77x10(17) cm-3 were s tudied. All samples investigated show the in rho is-proportional-to T- 1/2 dependence in the temperature range below 1.5K. As thermistor mate rials NTD Ge-70:Ga samples are found to have more than factor of two h igher sensitivity than commonly used natural NTD Ge in the temperature range between 0.2K and 1K. Our results are compared with theoretical predictions for variable range hopping conduction.